In recent years, lithium secondary batteries (typically lithium-ion batteries), nickel hydrogen batteries and other secondary batteries have gained importance as vehicle-mounted power sources or power sources for personal computers and handheld devices. Lithium secondary batteries hold particular promise as high-output vehicle-mounted power sources because they provide high energy densities with low weight.
First and foremost, lithium secondary batteries used as motor drive power sources in electric vehicles (EV), hybrid vehicles (HV), plug-in hybrid vehicles (PHV) and other vehicles must be suited to high-rate (such as 10 C or more) charge and discharge. One way of satisfying this first requirement is to use a fine particulate compound for the positive active material. Recently, fine particulate positive active materials with an average primary particle diameter of less than 1 μm have come to be used. Such a fine particulate positive active material is suited to high-rate charge and discharge because it has a relatively large specific surface area. For example, Patent Document 1 below describes a positive active material for a lithium secondary battery, which is a particulate positive active material consisting of a composite metal oxide containing lithium and manganese, wherein the percentage of particles that remain in a state of primary particles without forming secondary particles is more than half of the total of all the composite metal oxide particles.
The second requirement of a lithium secondary battery used as a motor drive power source is high durability. That is, vehicular batteries are used over a long period of time while being charged and discharged at a high rate (with high output) under severe environmental conditions, which may include extreme temperature changes (such as low temperatures below −20° C. and high temperatures above 60° C.). Thus, they must be durable enough that the increase in internal resistance of the battery is controlled under such conditions of use. One way of satisfying this second requirement is to support the positive active material particles at high densities at a specific location (that is, in the positive active material layer) on the positive collector. An effective way of doing this is to raise the (percentage) content of binder in the positive active material layer.
However, when the (percentage) content of binder is increased, the (percentage) content of the positive active material is decreased proportionally, reducing the capacity of the battery. For example, in the technology described in Patent Document 1 above, it is expected that a large quantity of binder will be required so that the particulate positive active material, more than half of which consists of primary particles, does not peel (separate) from the positive electrode collector.
Patent Document 2 below discloses a secondary battery positive electrode, wherein the primary particles constituting the positive active material are bound with a water-soluble polymer binder to thereby form aggregates (secondary particles) of linked primary particles, and these secondary particles are then bound with each other and to the positive electrode collector with a fluororesin binder or rubber-based binder. The binding force of the positive active material layer is described as being improved with this configuration, but relatively large quantities of at least two different binders are required. Another document of prior art of this type is Patent Document 3 below for example. This document describes obtaining improved electron conductivity of the positive active material particles by mixing carbon fiber with the particulate lithium phosphoric acid transition metal compound constituting the positive active material.    Patent Document 1: Japanese Patent Application Laid-open No. 2003-203632    Patent Document 2: Japanese Patent Application Laid-open No. 2007-234277    Patent Document 3: Japanese Patent Application Laid-open No. 2008-117749